Pengtao Guo,Miao Chang,Tongan Yan,Yuxiao Li,Dahuan Liu

State Key Laboratory of Organic-Inorganic Composites,Beijing University of Chemical Technology,Beijing 100029,China

Keywords:Adsorption Light hydrocarbons Metal-organic framework Natural gas Selectivity Separation

ABSTRACT Metal-organic frameworks (MOFs) have great potentials as adsorbents for natural gas purification.However,the trade-off between selectivity and adsorption capacity remains a challenge.Herein,we report a pillared-layer metal-organic framework Ni(HBTC)(bipy) for efficiently separating the C3H8/C2H6/CH4 mixture.The experimental results show that the adsorption capacity of C3H8 and C2H6 on Ni(HBTC)(bipy) are as high as 6.18 and 5.85 mmol·g-1,while only 0.93 mmol·g-1 for CH4 at 298 K and 100 kPa.Especially,the adsorption capacity of C3H8 at 5 kPa can reach an unprecedented 4.52 mmol·g-1 and for C2H6 it is 1.48 mmol·g-1 at 10 kPa.The ideal adsorbed solution theory predicted C3H8/CH4 selectivity is as high as 1857.0,superior to most of the reported materials.Breakthrough experiment results indicated that material could completely separate the C3H8/C2H6/CH4 mixture.Therefore,Ni(HBTC)(bipy)is a promising material for separation of natural gas.

1.Introduction

As the global economy overgrows,the environmental problems caused by the rapid consumption of fossil energy become more and more serious,inducing the increasing demand for clean energy[1].Natural gas is regarded as one of the important clean energy sources,as a potential energy to replace traditional fuels because of its safety,convenience,high calorific value,and low CO2emission [2,3].Global natural gas consumption amounts reach to 2.97 trillion cubic meters per year for the moment and increases year by year [4,5].The main component of natural gas is CH4,with a small amount of C2H6and C3H8.Even though the amounts of C2H6and C3H8are small,they can affect the efficiency of CH4utilization,the safety and stability of natural gas during transportation [6,7].On the other hand,C2H6and C3H8are important chemical materials.For example,C2H6is an important raw material for producing C2H4,which is widely used in the production of plastics and rubber[8].C3H8plays an important role in the production of C3H6and energy supply[9].Therefore,it is necessary to develop technologies for the efficient recovery of CH4,C2H6and C3H8from natural gas to obtain high-purity single-component gases to make full use of such kind of resource.

The most common separation technique currently used in industry for C3H8/C2H6/CH4is low-temperature distillation,which is however energy-intensive and under harsh conditions,such as high pressure and low-temperature conditions [10-13].In contrast,pressure swing adsorption (PSA) separation has the advantages of mild operating conditions,high efficiency,simple operation,and low cost,and is considered as an effective method for the separation of C3H8/C2H6/CH4[12,14-16].The key is the adsorbent with excellent performances,while most of the traditional adsorbents such as molecular sieve and zeolite show low selectivities or low adsorption capacities up to now.Therefore,developing new adsorbents with high adsorption capacity and high selectivity is necessary for separation C3H8/C2H6/CH4mixture.

As a kind of porous materials with a periodic network structure,consisting of metal ions/clusters and organic ligands,metal-organic frameworks(MOFs)have been widely used in gas adsorption and separation [17-22],catalysis [23-27],and sensing [28-32].Especially,the diversity of inorganic and organic ligands,adjustable pore size,and modifiable framework surface of MOFs indicate their great potentials in the gas separation field[33-39].For natural gas,the concentration of C3H8and C2H6is about 5% and 10%respectively,thus the adsorbent with a high adsorption capacity of C2H6and C3H8in low-pressure regions is the key to realize the high efficiency separation of C3H8/C2H6/CH4[40].Unfortunately,most of the adsorbents reported for C3H8/C2H6/CH4separation exhibit low adsorption capacities of C3H8and C2H6at low pressures.For example,Zhanget al.constructed a supramolecular metal-organic framework,BSF-2,using [B12H11I]2-instead of[B12H12]2-[41].This MOF has the high selectivity (2609.0) for C3H8/CH4(50/50) and C2H6/CH4(50/50) (53.0),while the adsorption capacities for C2H6and C3H8are only 1.22 and 1.77 mmol·g-1at 298 K and 100 kPa.Especially at 5 kPa,the adsorption capacity of C3H8is only 0.77 mmol·g-1.Lvet al.reported that the C3H8adsorption capacity in CTGU-15 at 5 kPa and 298 K is only 0.39 mmol·g-1and the C3H8/CH4and C2H6/CH4selectivities are only 170.7 and 5.2 respectively,though it has an ultra-high C3H8adsorption capacity(12.13 mmol·g-1) at 100 kPa and 298 K [42].Considering the productivity,the low gas adsorption capacity in the low-pressure regions limit the separation performance for C3H8/C2H6/CH4in the practical applications for most of MOFs reported so far.Therefore,it is necessary to find material with high adsorption capacity for C3H8at 5 kPa,C2H6at 10 kPa,as well as high separation selectivity.

In this work,we report a pillared-layer MOF Ni(HBTC)(bipy)[43] for the separation of C3H8/C2H6/CH4.The adsorption and separation ability of Ni(HBTC)(bipy) on C3H8/C2H6/CH4mixture was systematically investigated by single-component adsorption isotherms and breakthrough experiment.The results show that it has an excellent C3H8and C2H6adsorption capacity at 298 K and 100 kPa,with the unprecedented C3H8adsorption capacity(4.52 mmol·g-1) at 5 kPa and 1.48 mmol·g-1for C2H6at 10 kPa.In addition,the selectivity of C3H8/CH4and C2H6/CH4predicted by the ideal adsorbed solution theory (IAST) can reach 1857.0 and 27.5,which is among the highest values reported in literature.Breakthrough experiments demonstrate that Ni(HBTC)(bipy) can completely separate the ternary mixture of C3H8/C2H6/CH4.

2.Materials and Methods

2.1.Material synthesis

Nickel(II) nitrate hexahydrate (Ni(NO3)2·6H2O,99.9%),1,3,5-benzene tricarboxylic acid (H3BTC,95%),4,4′-bipyridine (bipy,98%),andN,N-dimethylforma-mide (DMF,99.9%)were purchased from J&K(China).All the reagents and solvents were commercially available and directly used without additional purification.Ni(HBTC)(bipy) was synthesized according to the method reported in the literature [43].Ni(NO3)2·6H2O (3.25 mmol,945.0 mg),H3BTC (2.75 mmol,578.0 mg) and bipy (2.75 mmol,429.5 mg)were dissolved in a 200 ml mixture of DMF/MeOH (1:1) under ultrasonication for 10 min.The resulting homogeneous solution was then transferred to a 200 ml glass bottle.The tightly sealed glass bottle was heated in an oven at 343 K for 2 days.The obtained Ni(HBTC)(bipy) crystals were collected and washed several times with DMF and then air-dried at room temperature.

2.2.Characterizations

Powder X-ray diffraction (PXRD) data were recorded on a D8 Advance X-ray diffractometer (Bruker,Germany) equipped with a Cu sealed tube(λ=0.0154178 nm).The pore size,BET surface area and gas sorption measurement were collected using a Quantachrome Autosorb-IQ instrument (Quantachrome,USA).GA Q50(TA Instruments,USA) carried out thermogravimetric analyses(TGA) with air atmosphere (10 ml·min-1,from 298 to 900 K with the rate of 10 K·min-1).The morphologies of materials were characterized by a scanning electron microscope (SEM,S4700 Hitachi,Japan) with an accelerating voltage is 20.0 kV.The effluent gas in the adsorption bed was analyzed using a multicomponent mass spectrometer (BSD-MAB,Beishide Instrument,China).

2.3.Gas adsorption experiment

To investigate the adsorption and separation performance of the mixed C3H8/C2H6/CH4components,the adsorption isotherms of CH4,C2H6,and C3H8were measured at 273 and 298 K using an ASAP 2020 instrument (Micromeritics,USA).A powder sample of at least 1.0 g was placed in the sampling tube for the measurements.To completely remove the guest molecules,the sample was degassed at 423 K under vacuum for 12 h.During the measurements,the adsorption pressure was varied from 0.0 to 100 kPa,and a water bath was used to maintain the system temperature.Equilibrium was held for 15 min at each point on the isotherm,and adsorption data were collected.For the adsorptiondesorption cycle measurements of C3H8and C2H6,the samples were vacuum degassed immediately after the adsorption measurements were completed,followed by the next adsorption-desorption cycle.

2.4.Calculation of isosteric heat of adsorption

The coverage-dependent isosteric heat of adsorption of CH4,C2H6,and C3H8was calculated using the Clausius-Clapeyron equation:

whereRrepresents the molar gas constant (8.314 J·K-1·mol-1),Pirepresents the pressure of isothermi,andTirepresents the temperature.

2.5.Calculation of selectivity

Based on the experimental data of single-component adsorption,the selectivity of material was calculated by IAST.Singlecomponent adsorption isotherms were first fitting by dual-site Langmuir-Freundlich model.Considering the practical situations,the molar ratio of C3H8/CH4and C2H6/CH4in the gas mixture are both set to be 50:50.The dual-site Langmuir-Freundlich model is expressed as below:

whereNis the equilibrium adsorbed amount of the adsorbate in an adsorbent(mmol·g-1),andare the saturation uptakes on site 1 and site 2(mmol·g-1),bAandbBare the affinity coefficients of site 1 and site 2 (kPa-1),andVAandVBare the corresponding deviations from ideal homogeneous surfaces.

Sijrepresents the ideal selectivity of material which can be defined as:

whereqiandqjrepresent the adsorbed quantity,andyiandyjrepresent gas molar fractions of componentsiandj,respectively.

2.6.Breakthrough experiments

To evaluate the dynamic separation performance of Ni(HBTC)(bipy),breakthrough experiments were performed using a mixed feed simulating the composition of natural gas (C3H8/C2H6/CH4).In this work,1.0 g of Ni(HBTC)(bipy) powder sample were heated for 12 h under dynamic vacuum at 423 K to activate the material and then loaded into a stainless steel column (the length is 9.0 cm,and the inner diameter is 0.6 cm).The voidage of the packed bed is about 0.52.Before breakthrough measurements,the bed containing the sample was purged in situ with a flow rate of 30 ml·min-1for 2 h at 423 K under a helium atmosphere(＞99.99% purity).A mixture of C3H8/C2H6/CH4(5/10/85) was then fed into the bed with a flow rate of 10 ml·min-1at 298 K and 100 kPa.The component concentrations in the exhaust gas were continuously monitored by the multicomponent mass spectrometer.

2.7.Theoretical calculations

The model system used to analysis van der Waals (vdW) interaction was taken from experimental crystal structure [44],from which the framework fragment and guest molecule was intercepted for clearly showing the distances.In DMol3of Materials Studio,the positions of H atoms for framework fragment passivation are optimized by using a B3LYP functional based DFT calculation method [45].Combining the double numerical plus d-functions(DND) [46] basis set,the generalized gradient approximation(GGA) and Perdew-Burke-Ernzerh (PBE) [47] functionals were used for calculations.The convergence criterion for selfconsistent field (SCF) calculation was 2.6252 × 10-2kJ·mol-1in energy,and the thermal smearing method of 13.126 kJ·mol-1was used to accelerate the convergence.

3.Results and Discussion

3.1.Characterizations

The structure of Ni(HBTC)(bipy) is shown in Fig.1.Twodimensional(2D)layers are formed by coordinating metallic nickel and H3BTC ligand.The adjacent 2D layers are bridged by aromatic bipy ligand to form a three-dimensional (3D) porous framework.As shown in Fig.2(a),the experimental PXRD patterns match well with the simulated ones,indicating the successful preparation with high-purity and crystallinity.From the SEM images in Fig.S1 (Supplementary Material),the crystals are hexagonal in shape with a uniform and regular geometric structure,which is due to the formation of well-crystallized Ni(HBTC)(bipy) during the synthesis process.Fig.2(a) also shows that the structure of material remains almost intact when being placed in wet air for 3 days,indicating the excellent stability.

N2adsorption-desorption isotherm of Ni(HBTC)(bipy) was measured at 77 K,as shown in Fig.2(b).The results indicate that the material exhibits substantial adsorption of N2at low pressures,and the desorption isotherm shows no hysteresis loop,indicating that the material is microporous.Based on the N2adsorption isotherm,the BET specific surface area and pore volume of Ni(HBTC)(bipy) are 1525 m2·g-1and 0.62 cm3·g-1,respectively (Table S1,see Supplementage Material).The Zeo++open-source resource package was used to calculate the pore size.It has two kinds of pores distributed around 0.64 and 0.74 nm,respectively.These results demonstrate that Ni(HBTC)(bipy) can better accommodate the C2H6and C3H8molecules,endowing it with the potential to separate C3H8/C2H6/CH4mixture.As shown in TGA curve in Fig.S2,Ni(HBTC)(bipy) undergoes two stages of weight loss.The first stage is about 32% weight loss until 420 K,which is due to the removal of the guest molecules (DMF and H2O molecules).In the second stage,from 537 to 730 K,the weight loss is about 47%,mainly due to decomposition and structural collapse of the material.These results indicate that Ni(HBTC)(bipy) can remain stable up to 537 K.

3.2.Gas adsorption performance of light hydrocarbons

To study the gas adsorption performance,the single-component adsorption isotherms of CH4,C2H6and C3H8in Ni(HBTC)(bipy)were measured at 298 and 273 K.As shown in Fig.3(a) and (b),the adsorption capacity decreases with the increase of temperature,indicating that the adsorption on Ni(HBTC)(bipy) is physical adsorption.The adsorption capacity increases with increasing carbon numbers(C3H8＞C2H6＞CH4).For C3H8and C2H6,the amounts are 6.18 and 5.85 mmol·g-1at 298 K and 100 kPa,respectively,while it is only 0.93 mmol·g-1for CH4.It is worth noting that the adsorption of C3H8increases sharply at the range of 0-5 kPa,indicating that the interaction between C3H8and the framework is stronger than those of C2H6and CH4.It can reach 4.52 mmol·g-1at 5 kPa.Since the concentration in natural gas is very low (about 5%),the C3H8adsorption capacity at the low-pressure range is crucial for purifying natural gas.As shown in Fig.3(d),the adsorption capacity of C3H8at 5 kPa in Ni(HBTC)(bipy)exceeds those in other reported typical adsorbents for C3H8separation.Moreover,Ni(HBTC)(bipy) has excellent C3H8adsorption capacity at 298 K and 100 kPa,which is significantly higher than MIL-142A(5.32 mmol·g-1) [48],FJI-C4 (3.19 mmol·g-1) [49],UPC-98(4.35 mmol·g-1) [50],and BSF-2 (1.77 mmol·g-1) [41].Although CTGU-15 exhibited a very high C3H8adsorption capacity(12.13 mmol·g-1) [42],the C3H8adsorption capacity at 5 kPa is only 8.6% of that in Ni(HBTC)(bipy).Five adsorption-desorption cycles of the material for C2H6and C3H8at 298 K(Fig.3(c))indicate that the material has excellent regenerability,which is beneficial for the use in practical applications.

Fig.1.The crystal structure of Ni(HBTC)(bipy) viewed along c axis (a) and b axis (b).

Fig.2.PXRD patterns (a) and N2 adsorption-desorption isotherm (b) of Ni(HBTC)(bipy) at 77 K (inset:pore size distribution).

Fig.3.Experimental adsorption isotherms of CH4,C2H6 and C3H8 in Ni(HBTC)(bipy)at 273 K(a)and 298 K(b),cyclic regeneration experiments for C2H6 and C3H8 adsorption(c),and the comparison of C3H8 uptake at 5 kPa (d).

To evaluate the affinity of the framework,the adsorption heats(Qst) of C3H8/C2H6/CH4on Ni(HBTC)(bipy) was estimated by Clausius-Claperon equation based on the single-component absorption isotherms.As shown in Fig.S3,the values ofQstfor C3H8,C2H6and CH4are 50.45,25.71 and 18.04 kJ·mol-1at the zero coverage.The trend of adsorption heats was C3H8＞C2H6＞CH4,reflecting the stronger interaction of C3H8and C2H6with the framework than CH4,which is in consistent with the results of single-component absorption isotherms.

3.3.Separation performance of light hydrocarbons

To investigate the separation performance,the IAST model[51]was applied to predict the separation selectivity of C3H8/CH4and C2H6/CH4in Ni(HBTC)(bipy).At the first step,the dual-site Langmuir-Freundlich model (DSLF) was used to fit the adsorption isotherms of each pure component at 298 K.The fitted parameters and correlation coefficients are shown in Table S2 and Fig.S4,with the correlation coefficientR2up to 0.999,indicating that the DSLF model can well describe the adsorption behavior.

Fig.4.(a)The IAST-predicted selectivity of Ni(HBTC)(bipy)for C2H6/CH4(50/50)and C3H8/CH4(50/50).(b)Comparison of C3H8 uptake at 5 kPa and IAST selectivity for C3H8/CH4 (50/50) mixture at 298 K (JXNU-4:5/95,PAN-m3/p1:10/90,ZnP-CTF-600:unspecified molar ratio).

IAST-predicted selectivities of C2H6/CH4(50/50) and C3H8/CH4(50/50)in Ni(HBTC)(bipy)are shown in Fig.4(a).It can be seen that the selectivity increases with the increase of pressure,probably because of the high polarizability of C3H8((62.9-73.7) × 10-25cm3)compared to CH4(25.93×10-25cm3)(Table S3).The interaction between the C3H8molecules adsorbed on the framework and the unadsorbed C3H8is stronger than that of CH4.The selectivity for C3H8/CH4is much higher than that of C2H6/CH4under the same conditions,mainly due to the more significant interaction of C3H8with the framework than C2H6and CH4,as reflected from the single-component adsorption isotherms.When the pressure reaches 100 kPa,the selectivity of C3H8/CH4is as high as 1857.0,and the selectivity of C2H6/CH4is 27.5.To the best of our knowledge,the selectivity of C3H8/CH4exceeded all the reported values in other porous materials except for BSF-2,such as UTSA-35A(80.0) [52],Iso-MOF-4 (80) [53],FJI-C4 (293.4) [49],PCN-224(609.0) [54] and MIL-142A (＞1300) [48].For BSF-2,though the selectivity of C3H8/CH4is high,the adsorption capacity of C3H8is relatively low due to the lower specific surface area and pore volume.In contrast,the adsorption capacity of C3H8in Ni(HBTC)(bipy)is 4.3 times higher than that of BSF-2 in the low-pressure region(5 kPa) at 298 K,and 3.4 times at 100 kPa.From Table S4,it can be seen that CTGU-15 has a surprising adsorption capacity of C3H8adsorption (12.13 mmol·g-1),while the C3H8/CH4selectivity is only 170.1,only 9.2% of that in Ni(HBTC)(bipy).Moreover,the C3H8adsorption capacity(4.52 mmol·g-1)is 11.5 times higher than that of CTGU-15 (0.39 mmol·g-1) at 5 kPa.To simulate the actual situation of natural gas,we also calculated the selectivity of the C3H8/CH4(5/85) and C2H6/CH4(10/85) gas mixtures.As shown in Fig.S5,the selectivities are 317.5 and 16.9,which exceed those in most of the material reported so far (Table S4).These results indicate that Ni(HBTC)(bipy) has excellent C3H8adsorption capacity and high C3H8/CH4selectivity,which effectively overcomes the trade-off between adsorption capacity and selectivity of the adsorbent.These results indicate that this MOF has a good separation ability of C3H8/C2H6/CH4and can separate CH4from the mixture of C3H8/C2H6/CH4,as a potential adsorbent for the purification of natural gas.

Fig.5.Adsorption binding sites calculated by DFT:(a) CH4,(b) C2H6,(c) C3H8.

The high selectivity of Ni(HBTC)(bipy) for C3H8/C2H6/CH4separation is mainly attributed to Ni(HBTC)(bipy) is an ultramicroporous material with a pore size that can well accommodate C3H8molecules (C3H8:0.43-0.51 nm).This ultramicroporous MOF can produce stronger potential energy overlap with C2H6and C3H8.Since the pore channel of the material consists of the lowpolarity benzene ring and pyridine rings,it can form strong interactions with molecules with high polarizabilities.The polarizabilities of C3H8,C2H6,and CH4are (62.9-63.7) × 10-25,(44.3-44.7) × 10-25and 25.93 × 10-25cm3,respectively.Thus,C2H6and C3H8exhibit stronger interactions with the framework [48-49,55-56].

3.4.Microscopic mechanism analysis

In view of the excellent C3H8/CH4separation ability and C3H8,C2H6adsorption capacity of Ni(HBTC)(bipy),DFT calculations were carried out to understand the interaction of CH4,C2H6and C3H8with the framework.With C3H8,C2H6and CH4molecules loaded into the Ni(HBTC)(bipy) framework,it is found that van der Waals(vdW)interaction plays the main role through geometry optimization[57].As shown in Fig.5,the distances between the H-atoms of C3H8,C2H6,CH4and the C-H atoms of the organic ligands are in the ranges of 0.249-0.270,0.272-0.305 and 0.310-0.354 nm,respectively.These values are less than the vdW interaction range of 0.3400 nm reported in the literature[58],indicating that the multiple vdW interactions between adsorbent and adsorbate are formed with a strong synergistic effect.

3.5.Breakthrough experiments

Excellent C3H8and C2H6adsorption capacity and C3H8/CH4separation selectivity encourage us to further verify the ability of Ni(HBTC)(bipy) for dynamically separating C3H8/C2H6/CH4mixture.The corresponding breakthrough experiments of C3H8/C2H6/CH4(5/15/85)were conducted at 298 K and 100 kPa,and the composition is similar to that in the actual natural gas.It can be seen from Fig.6 that CH4(～1.7 min)first elutes the breakthrough column,followed by C2H6(～20.7 min),and finally C3H8(～130.7 min).Breakthrough time for C3H8is the longest,indicating C3H8has the strongest interaction with the framework and the highest adsorption capacity of C3H8,followed by C2H6and CH4.This is in consistent with the calculated heat of adsorption and the measured results of the adsorption isotherms.The amount of C3H8and C2H6captured from the C3H8/C2H6/CH4mixture is about 3.0 and 1.21 mmol·g-1,which is lower than the static adsorption capacity(4.52 mmol·g-1at 5 kPa for C3H8,and 1.48 mmol·g-1at 10 kPa for C2H6),indicating the competitive adsorption of C3H8,C2H6and CH4on Ni(HBTC)(bipy).

Fig.6.The breakthrough curves for C3H8/C2H6/CH4 (5/10/85) mixture through Ni(HBTC)(bipy) packed column at 298 K and 100 kPa.

4.Conclusions

In summary,we investigated the ability for the purification of natural gas using a Ni-based pillared-layer MOF Ni(HBTC)(bipy).Due to the ultramicroporous feature of this material with suitable pore size,the strong potential energy overlap can be obtained for C3H8and C2H6.As a result,it exhibits a super high selectivity of C3H8/CH4(up to 1857.0),and C2H6/CH4selectivity (27.5).Meanwhile,the C3H8adsorption capacity can reach 4.52 mmol·g-1at 5 kPa,exceeding those in all the reported materials,which is beneficial for the efficient separation of C3H8/C2H6/CH4mixture with low C3H8concentrations.Therefore,Ni(HBTC)(bipy)can effectively balance the adsorption capacity and selectivity.The breakthrough experiments confirmed that the C3H8/C2H6/CH4mixture could be entirely separated under ambient conditions.In combination of the excellent thermal stability and regenerability,Ni(HBTC)(bipy)shows great promising in natural gas purification.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The financial support of the National Natural Science Foundation of China (No.21978005) is greatly appreciated.

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2021.08.011.